I. Field of the Invention
The present invention relates to wireless communication systems. More specifically, the present invention relates to a novel and improved system and method of power control in a wireless communications system.
II. Related Art
Wireless communication networks are enjoying notable popularity in all aspects of business, industry and personal life. As such, portable, hand-held communication devices have experienced widespread growth in recent years. Portable devices such as cellular phones are now commonplace with business and personal users alike. Additionally, advanced systems, such as satellite communications systems using portable, hand held and mobile phones, are currently being deployed.
In wireless communication systems, signals are subject to fading. Fading occurs when environmental factors diminish the power of a signal during its transmission from transmitter to receiver. One measurement that quantifies fading is the signal-to-noise ratio (SNR) of the received signal as measured at the receiver. Systems have been developed to adjust the transmitted power of the signal to compensate for fading. One such system is known as xe2x80x9csingle-loopxe2x80x9d power control.
In a single-loop power control system, the receiver monitors the SNR of the received signal and sends commands to the transmitter to adjust the transmitted power so as to maintain a specified xe2x80x9cthresholdxe2x80x9d SNR at the receiver. Conventional single-loop power control systems generally employ two or three types of such commands. One type of command instructs the transmitter to increase the transmitted power. Another type of command instructs the transmitter to decrease the transmitted power. The amount by which the transmitted power is increased or decreased in response to such a command is referred to as the xe2x80x9cgainxe2x80x9d of the loop. In some systems, a third type of command is used to instruct the transmitter to maintain the transmitted power at the current level.
Single-loop power control works well in an environment with slow fading. In slow fading, there is no substantial fading during the time required for a power control command to reach the transmitter and the resulting signal-to-noise ratio to be measured at the receiver, known as the xe2x80x9cperiodxe2x80x9d of the loop. One example of a slow fading environment is one having only thermal noise as signal interference.
However, in a signal environment with medium-speed fading, single-loop power control is inadequate. In medium-speed fading, there is substantial fading during a single loop period. One example of a medium-speed fading environment is where the transmitter or receiver is moving rapidly past stationary obstructions, causing rapid changes in signal attenuation. In such a medium-speed fading environment, the threshold SNR may be insufficient to ensure signal quality. This is because the loop is too slow to respond to the rapid variations in the SNR of the received signal.
In digital communication systems, the adequacy of the threshold SNR can be quantified by the ratio of information bits received in error to the total number of bits received. This ratio is generally computed repeatedly for each frame. The ratio thus computed is known as the xe2x80x9cframe error ratexe2x80x9d (FER) of the signal. One type of system developed to address this problem is known as a xe2x80x9cdouble-loopxe2x80x9d power control system.
In a double-loop power control system, the single-loop power control system described above is used as the xe2x80x9cinnerxe2x80x9d loop. The SNR threshold used by the inner loop is modified by an xe2x80x9couterxe2x80x9d loop based on the FER of the received signal. For example, when the FER rises above a predetermined PER threshold, the threshold SNR is increased by a fixed, predetermined amount. This process continues until the FER falls below the FER threshold.
One consideration in double-loop power control systems is the selection of the magnitude of the fixed gain employed by the inner loop. The selection of this gain is a trade-off between two conflicting considerations. In a medium-speed fading environment, rapid loop response is required. This consideration argues for a large inner-loop gain. With a large inner-loop gain, fewer loop periods are required to change the threshold SNR by a large amount. However, in a slow fading signal environment, a large gain will result in large SNR oscillations about the threshold SNR. These oscillations waste transmitter power. Thus a fixed inner-loop gain is not suitable for applications in which the signal will experience both fast fading and slow fading.
Furthermore, fixed gain systems experience difficulty in fast fading signal environments. In fast fading, the SNR experiences several large oscillations within a single outer-loop period (that is, the time required to adjust the SNR threshold based on one or more FER measurements). Fast fading oscillations are typically on the order of hundreds of hertz. In such an environment, the response time of the inner loop is no longer important because the inner loop cannot possibly keep up with the fading. What is needed is a double-loop power control system where the inner-loop gain can be varied to suit the speed of the fading.
The present invention is an apparatus and method for adjusting the power of a signal sent by a transmitter to a receiver to compensate for fading in a wireless communications system. In one embodiment, the method includes the steps of measuring, at a first station, a signal-to-noise ratio of a signal transmitted by a second station; adjusting a transmitted signal power of the signal as a function of a loop gain, the signal-to-noise ratio, and a signal-to-noise ratio threshold; measuring, at the first station, a signal quality of the received signal; adjusting the signal-to-noise ratio threshold as a function of the signal quality and a signal quality threshold; measuring, at the first station, a fading rate of the signal; and adjusting the loop gain as a function of the fading rate and a fading rate threshold.
In one embodiment, the method includes the steps of measuring, at a first station, a signal-to-noise ratio of a signal transmitted by a second station; adjusting a transmitted signal power of the signal as a function of a loop gain, the signal-to-noise ratio, and a signal-to-noise ratio threshold; measuring, at the first station, a signal quality of the received signal; adjusting the signal-to-noise ratio threshold as a function of the signal quality and a signal quality threshold; measuring, at the second station, a fading rate of a further signal transmitted by the first station; and adjusting the loop gain as a function of the fading rate and a fading rate threshold.
One advantage of the present invention is that it mitigates the effects of fast fading.